Effect of flow direction on heat transfer characteristics of supercritical CO2 in a heating serpentine micro-tube

The combined effect of gravitational buoyancy, centrifugal force and centrifugal buoyancy on the heat transfer of supercritical CO2 in a serpentine micro-tube under different flow directions is numerically studied. The comprehensive heat transfer performance in the four flow directions is vertical upward flow (VU), horizontal flow vertical bend (HV), horizontal flow horizontal bend (HH) and vertical downward flow (VD) in ascending order. Considering the angle between gravity and centrifugal force and the angle between gravity and local flow direction, Richardson number can better reflect the effect degree of buoyancy under different flow directions. In VU, VD and HV, the gravitational buoyancy at low mass flux can weaken the local heat transfer deterioration at the turning point, but the gravitational buoyancy at high mass flux can only weaken the local heat transfer deterioration at the turning point when the heat transfer of tube section is enhanced. In HH, the local flow di-rection, gravity and centrifugal force are perpendicular to each other, and the location of local heat transfer deterioration at the turning point is affected by the ratio of centrifugal force to gravity.